chore: update @prettier/plugin-xml to v2.0.0

[kenji-miyake]

https://github.com/prettier/plugin-xml/releases/tag/v2.0.0
https://github.com/prettier/prettier/releases/tag/2.6.0

[kenji-miyake]

It seems there were no behavior changes.

~/ghq/github.com/autowarefoundation/autoware_launch main
❯ pre-commit try-repo https://github.com/tier4/pre-commit-hooks-ros prettier-launch-xml -a --ref update-prettier-xml
===============================================================================
Using config:
===============================================================================
repos:
-   repo: https://github.com/tier4/pre-commit-hooks-ros
    rev: update-prettier-xml
    hooks:
    -   id: prettier-launch-xml
===============================================================================
[WARNING] The 'rev' field of repo 'https://github.com/tier4/pre-commit-hooks-ros' appears to be a mutable reference (moving tag / branch).  Mutable references are never updated after first install and are not supported.  See https://pre-commit.com/#using-the-latest-version-for-a-repository for more details.  Hint: `pre-commit autoupdate` often fixes this.
[INFO] Initializing environment for https://github.com/tier4/pre-commit-hooks-ros.
[INFO] Initializing environment for https://github.com/tier4/pre-commit-hooks-ros:@prettier/plugin-xml@2.0.0,prettier@2.6.0.
[INFO] Installing environment for https://github.com/tier4/pre-commit-hooks-ros.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
prettier launch.xml......................................................Passed